Contours Lecture Notes

Facts about Contours

• Contours are imaginary lines that do not physically exist on the ground.
• Contours are represented by brown lines on a 1:50,000 scale map.
• Contour lines never intersect or cross each other.
• Contour lines are continuous and closed, unless they extend to the edge of a map.
• Contour line numbers are printed upright, upside down, or sideways to indicate the direction of higher and lower contour lines.
• Closely spaced contour lines indicate a steep slope, while widely spaced lines represent a gentle slope.
• Index contours are bolded to facilitate quick altitude range calculations.
  • On a 1:50,000 map, index contours are 100m apart vertically.
  • On a 1:10,000 orthophoto map, they are 20m apart.
• The contour interval is the altitude difference between successive contours.
• Subcontours, shown as fine, broken, brown lines, are used in very flat regions where a 20m contour interval is too large to effectively show relief features.

Types of Slopes

• Uniform Slope
  • A uniform slope is consistent throughout.
  • Also referred to as an even slope.
  • Can be either steep or gradual.
  • Contour lines are spaced at roughly equal distances.
• Gradual Slope
  • Also called a gentle slope.
  • Contour lines are far apart.
  • Altitude changes minimally over a long distance.
• Steep Slope
  • Contour lines are very close together.
  • Significant altitude variation occurs over short distances.
• Terraced Slope
  • Also known as a stepped slope.
  • Alternates between steep and gentle sections.
  • Contour lines alternate between being close together and farther apart.
  • Common in the Karoo region, where horizontal layers of hard rock create steeper slopes.
• Convex Slope
  • The bottom of the slope is steep, transitioning to a gentler slope towards the top, forming an outward curve.
  • Contour lines are closer together at the bottom and become more separated moving upwards.
• Concave Slope
  • The reverse of a convex slope, forming an inward curve.
  • Contour lines are spaced far apart at lower altitudes and close together at higher altitudes.
• Vertical Slopes
  • Also known as a cliff.
  • Identified by overlapping contour lines.

Spurs and Valleys

• Spurs are low mountain ridges often divided by incised valleys.
• A spur is indicated by contours forming a “V” shape, with the base of the “V” pointing towards lower ground.
• A valley is indicated by the “V” shape of contours pointing towards higher ground.

Watershed

• Also known as a divide.
• A ridge or higher ground that separates two river systems or drainage basins.